Field
This invention relates generally to a system and method for providing non-linear transmitter pre-coding for a transmit signal in a satellite communications system so as to reduce non-linear distortions in the signal and, more particularly, to a system and method for providing non-linear transmitter pre-coding for a transmit signal in a satellite communications system so as to reduce non-linear distortions in the signal, where the pre-coding includes combining dirty paper coding and non-linear high power amplifier (HPA) modeling, such as Volterra series modeling, of the transmit signal.
Discussion
Satellite communications is seeing a growing demand for greater throughput and transponders with more DC power efficiency. To provide greater spectral efficiency, satellite communications systems often employ modern coding and modulation, which is best exemplified by the known digital video broadcasting (DVB)-S2 standard and protocol. Code concatenation is an effective way to achieve large coding gains while maintaining the decoding complexity to be manageable. However, ever since the discovery of “turbo-codes” it has been widely acknowledged that the iterative processing techniques are not limited to conventional concatenated error correction codes, and the so-called “turbo principle” is more generally applicable to many other components found in modern digital communications. One such example is iterative de-mapping and decoding in coded modulation communications systems. Coded M-ary amplitude phase shift keying (APSK) is now the “de-facto” bandwidth-efficient modulation technique for digital satellite communications.
In order to meet the radiated power demands necessary for the signal transmission distances, satellite communications systems typically employ high power amplifiers (HPAs), such as traveling-wave tube amplifiers (TWTAs) or solid-state power amplifiers (SSPA). To provide high throughput and increased efficiency, these HPAs often operate at or near their saturation level, which typically results in significant non-linear distortions of the transmitted signal that has a reverse effect on the throughput and performance of the communications channel. M-ary APSK provides a power-efficient and spectral-efficient solution with its inherent robustness against highly non-linear distortion. However, the implementation of satellite communications channels still provides significantly different design challenges from traditional terrestrial channels due to their dominant non-linear behavior.
In a typical wireless digital signal transmitter for satellite communications or otherwise, the transmitter includes digital components that convert the digital bits at a particular point in time into an in-phase and quadrature-phase symbol constellation for transmission. However, these components include memory in that the symbol transmitted at a particular point in time includes artifacts from previously transmitted symbols that interfere with the desired transmit signal. Particularly, each symbol transmitted at a particular point in time will include significant interference from the previous symbols, where the actual transmitted symbol at a particular point in time includes the desired signal and interference from the signal in previous time slots. Further, the channel impulse response in the channel creates inter-symbol interference (ISI) in the signal, also causing signal distortions.
For those digital communications systems that cause non-linear signal distortions in the channel, some type of signal correction is typically required. Transmitter pre-coding (TPC) is a known technology in the communications art where the transmitter produces an estimate of the forward channel that is used to remove the anticipated non-linear distortions before the signal is transmitted so that the complexity associated with signal demodulation is provided in the transmitter and not in the receiver. One known TPC technique is referred to in the art as dirty paper coding, which relies on the concept that if the transmitter understands the forward communications channel, the transmitted signal can be altered so that when it is transmitted over the channel, the pre-coding compensates the interference of the channel so that the desired signal is clean when it is received by the receiver. In order to pre-code each symbol that is transmitted so that it only includes the desired signal symbol component, the transmitter subtracts the combined effect of the symbol of the previous time samples for the current symbol being transmitted so that only the desired symbol is actually transmitted.
For transmitter pre-coding in a typical digital subscriber line (DSL) communications system, the transmitter generally only needs to learn the channel one time at the beginning of the wireless transmission session because the channel will remain substantially constant for a relatively long period of time. For example, the transmitter may transmit a pilot tone to the receiver and based on how that signal changes in the channel will allow the transmitter to modulate the signal using a suitable pre-coding technique. However, for cellular wireless transmission, the ability to learn the forward channel (down-link) is much more complicated because the channel is constantly changing as a result of, for example, movement of the receiver, for example, a user terminal. In a typical cellular communications system, the cell phone receiver constantly transmits a feedback signal back to the transmitter in the base station at a different frequency (up-link) than the received signal, which is used for pre-coding purposes. However, by the time the transmitter receives the forward channel estimate signal from the receiver, the receiver has likely moved to a different location for the next transmission, where the previously received forward channel estimate may no longer be accurate. For satellite communications signals, the forward channel typically is not changing very rapidly, where forward channel estimates can be more accurately obtained.
Various techniques are known in the art for providing transmitter pre-coding in satellite communications systems. One known TPC technique is referred to as symbol pre-distortion without memory that primarily focuses on the non-linear effects caused by the HPA. However, this technique has limited performance and throughput. A second known TPC technique is referred to as symbol pre-distortion with memory, which also primarily focuses on the non-linear effects caused by the HPA. A third known technique is referred to as linear pre-equalization, which primarily focuses on removing channel generated distortion where the complexity depends on the forward channel.